This invention relates to circuits for recharging electrochemical cells and is particularly applicable to cells to which damage may result from overcharging. Where a plurality of cells are connected in series, various cells may self-discharge at different rates than others. Consequently, a series-connected charging source of constant current level will apply too high a voltage across those cells that first become fully charged. In some cells, particularly the high-temperature molten salt cells with metal sulfides as positive electrode materials, structural components of the cell may enter into undesired electrochemical and corrosive reactions. In other cells with aqueous electrolyte, water can be lost by electrolysis to H.sub.2 and O.sub.2.
Various high-temperature electrochemical cells with molten salt electrolyte have been described in U.S. Pat. Nos. 3,887,396 to Walsh et al., entitled "Modular Electrochemical Cell", June 3, 1975; 3,933,520 to Gay and Martino, entitled "Method of Preparing Electrodes with Porous Current Collector Structures and Reactants for Secondary Electrochemical Cells", Jan. 20, 1976; and 3,941,612 to Steunenberg et al., entitled "Improved Cathode Composition for Electrochemical Cell", Mar. 2, 1976. Other cell types are also illustrated in copending patent applications Ser. No. 636,882 by Kaun, entitled "Porous Carbonaceous Electrode Structure and Method for Secondary Electrochemical Cell", filed Dec. 2, 1975, and Ser. No. 642,438 by Roche et al., entitled "Calcium Alloy as Active Material in Secondary Electrochemical Cell", filed Dec. 19, 1975. Each of these patents and patent applications is assigned to the assignee of the present application.
The cells illustrated in these applications employ metal chalcogenides such as iron sulfides, copper sulfides, cobalt sulfides, and nickel sulfides as positive electrode materials opposite to negative electrodes that include as active materials the alkali metals, the alkaline earth metals and alloys of these metals, for example, lithium, lithium-aluminum, lithium-silicon, calcium, calcium-aluminum, calcium-magnesium and calcium-silicon. High-energy cells of these types operate effectively with molten salt electrolytes including compositions of the alkali metal halides and alkaline earth metal halides. Typical electrolytes are LiCl--KCl, LiF--LiCl--KCl and CaCl.sub.2 --NaCl.
Battery-charging circuits for secondary electrochemical cells of these types require close control of the charging voltage to prevent electrochemical degradation of the structural components within the cell. For example, in a cell using FeS as positive electrode material and iron or iron-base alloys as current collector and other structural components, the upper voltage level that can be applied to an individual cell is about 1.63 volts. The equilibrium voltage, that is, the open-circuit voltage at full charge, for the Li--Al/FeS cell is about 1.33 volts. For the Li--Al/FeS.sub.2 cell with molybdenum as a current collector, the upper level is about 2.1 volts with an equilibrium voltage of about 1.77 volts. Charge voltages must be above the equilibrium voltage to obtain full charge. However, charging schemes that impose voltage levels above the upper limit to an individual cell may result in an electrochemical attack by the electrolyte onto the cell's structural components.
Ordinarily batteries of these cells will involve a plurality of series-connected cells in order to obtain desired voltage levels. Recharging merely by imposing a controlled voltage or current through such a series-connected battery will result in excessive voltage on some cells before others are fully charged. Individual cells in a more fully charged state may be subjected to voltage levels that result in electrochemical degradation of cell structural components. Such an imbalance in the state of charge of individual cells can result from variations in self-discharge rates during use.
Battery systems including aqueous electrolyte such as the lead-acid battery have some inherent overcharge protection from electrolysis of water. Although this offers some protection to cell components, cell life may be shortened from loss of electrolyte. In addition, it may be desirable in some applications, such as in large banks of batteries for off-peak power storage, to seal the cells to eliminate the need for water replenishment. Consequently, the improved charging system of the present invention may also have use with batteries having aqueous electrolyte.
Therefore, it is an object of the present invention to provide an improved battery-charging system for use with a series-connected plurality of cells.
It is a further object to provide an uncomplicated system for conveniently recharging a plurality of series-connected cells with periodic equalization of cell charge.
It is also an object to provide an improved method for charging and equalizing a plurality of series-connected electrochemical cells.